Known in the art is an apparatus for detecting engine knock by means of a cantilevered reed oscillator made of a piezoelectric material which may be tuned to the knock frequency of the engine. A detector of this type is highly sensitive for detecting detonation or knocking in combustion chambers of the engine. However, the high sensitivity of such oscillators brings the disadvantage that the induced oscillation continues beyond the occurrence of the knock phenomenon which drives them. Accordingly, when the detectors are used in multi-cylinder engines, it is often impossible to assign the occurrence of knocking to a particular cylinder. Furthermore, the high sensitivity of the known oscillator causes it to be forced into oscillation by spurious events, for example the closure of the inlet valves, etc.